33-year old weathermatic valve won't turn sprinklers on

33 year-old black plastic Weathermatic valve, 4 screws on top, round solenoid, looks like "silver bullet" but has no ribs on top of molded case and brass flow control "cross' on top, diaphragm has brass conical weight at bottom.

I've tried disassembling, flushing the valve, cleaning the diaphragm, using the manual flow control lever to activate, loosening the solenoid, running without the solenoid plunger. Diaphragm has no holes other than the 2 tiny ones that belong there. I orient the diaphragm correctly.

When I try to run it (even with the little manual lever on the side) I hear what sounds like lots of water running. The most I get out of the sprinklers is a VERY tiny trickle.

I thought that air might be trapped. I rotated the manual control open and closed 2-3 times.

Strangely, when I expected a solenoid problem, I swapped the solenoids with the solenoid from a working valve. Now neither valve is working. I did not disassemble the second valve (other than the solenoid). Again, when I try to run the second one with the little manual lever on the side) I hear what sounds like lots of water running but no resulting water at sprinklers.

I have a working valve on the same system, so I don't expect that there are any water pressure issues.

I was thinking it was a hardened diaphragm, but don't understand how that would cause the issue with the 2nd valve. Diaphragm seems hard to find. I'm totally stumped. Thanks, in advance, for any help you can provide.

Exhaust port was clean. Although I didn't think a new diaphragm was needed, I was able to find a $7 replacement. Replacing the diaphragm did the trick. The old one didn't have a hole in it, but it was quite a bit harder than the new one.

Consider replacing all the old W*M diaphragms, which will reset the clock on a key system component.

And yes, the diaphragm was next in the chain of diagnostics. A pro would have known by the look and feel of the old one that it was time for a new one, but that look and feel does not translate into words, hence the solenoid and exhaust port test.